This invention relates to a driving technique for solid state imaging apparatus.
One of conventional solid state imaging apparatus is a combination of an optical sensor including a matrix array of photodiodes and an XY scanner implemented with field effect transistors.
The recent development in this field offers BBDs and CCDs (collectively referred to as "charge transfer type" hereinafter) which are almost free of spike-like noise originating from scanning pulses as experienced in the XY matrix cell structure. Both of the above mentioned types of imaging apparatus, however, require that the photodiodes in the optical sensor section be disposed in the same surface of a substrate as that where the field effect transistors for XY scanning or an electrode structure for charge transfer are formed, thus resulting that the efficiency of light utilization per unit area is one third through one fifth at the very best.
Several approaches to circumvent the above problem have been proposed--a solid state imager including in combination photoconductors instead of the photodiodes in the optical sensor portion and the XY matrix field effect transistors and a solid state imager including such photoconductors and the charge transfer electrode assembly. In any case, electrodes overlying the photoconductors are held at a DC potential.
An example of solid state imagers using the photoconductor coating is now discussed in greater detail.
Referring to FIG. 1, there is typically illustrated in cross section a unit of circuit elements of the charge transfer type disposed a silicon substrate. An n.sup.+ type region is buried in the p type semiconductor substrate 10 to set up diodes. A p.sup.+ type region 12 serves as a potential barrier for preventing injection of electrons from the n.sup.+ type region 11 in the case of CCD operation mode. An n.sup.+ type region 13 serves as a potential well in the case of BBD operation mode. Only either of these last two regions is required, depending upon a selected one of the two operation modes, CCD and BBD. Since both the CCD and BBD operation modes make use of charge transfer similarly, only the BBD mode using the n.sup.+ type region 13 is discussed in the following section.
A first gate electrode 14 has an overlap with the n.sup.+ region 11. A layer of insulator lying between the semiconductor substrate 10 and the first gate electrode 14 is a gate oxide film. An insulating layer 16 establishes electrical isolation between a first electrode 17 and the semiconductor substrate 10 and the first gate electrode 14. The first electrode 17 behaves as not only an electrode for a diode electrically connected to the n.sup.+ type region 11 but also one for a hole-stopping layer 18. A layer of photoconductor 19 is made of a composition (Zn.sub.1-x Cd.sub.x Te).sub.1-y (In.sub.2 Te.sub.3).sub.y with transparent electrode 20 overlying thereon. The transparent electrode is supplied with a positive DC potential V.sub.D from a voltage source 21. The potential supplied should not be constantly positive and in some cases may be negative, based upon the characteristics of the photoconductor layer.
The following description sets forth the reading of optical information on the conventional solid state imager structure as illustrated in FIG. 1.
FIG. 2 depicts the waveform of pulses for enabling the cell structure and variations in the potential at the first electrode 17.
If a pixel read pulse of a voltage V.sub.CH is applied to the first gate electrode 14 at instance t.sub.1, then the electrode 17 is charged to a potential V.sub.CR as seen in FIG. 2b, wherein V.sub.CR is the potential represented as follows: ##EQU1##
The reason why the potential V.sub.CR is defined as above will be discussed hereinafter.
V.sub.R is the reference voltage by which the cell structure is enabled in BBD mode upon application of a transfer pulse of a voltage V.sub..phi. to the first gate electrode and may be represented as follows: EQU V.sub.R =V.sub..phi. -V.sub.TB
V.sub.TB is the threshold voltage of an MOS transistor or a constitutional element of the BBD structure.
It should be noticed that those BBD elements are aligned in the direction T of transferring signal charges as shown in the drawings.
V'.sub.TR is the threshold voltage of the MOS transistor including a bias voltage to the substrate, the MOS transistor being built up by including the n.sup.+ type regions 11 and 13 and the first gate electrode 14.
Assuming that there is incident light 22 on the cell structure, it generates electron-hole pairs in the photoconductor layer 19 and the resulting holes proceed to the electrodes 17 and 20, reducing the potential at the electrode 17. The potential drops in proportion to the quantity of the incident light and especially drops gradually down to V.sub.S during a fram interval. Should V.sub.CH be applied again to the first gate electrode 14 at instance t.sub.2, the surface potential beneath that electrode increases so that transfer of electrons takes place from the n.sup.+ region 11 to the n.sup.+ region 13. The potential at the n.sup.+ region 11 consequently rises again and reaches V.sub.CR. A total of charges transferred to the n.sup.+ region 13 is dependent upon the intensity of the incident light.
Whereas the foregoing has set forth the structure of the solid state imaging unit comprising comprised of the optical sensing element and the first gate electrode 14, a mechanism by which electric signals converted from optical ones and read in the n.sup.+ region 13 are transported to an output stage by means of self-scanning will now be explained in detail.
FIG. 3 is a plan view illustrating in a single-dimensional manner the solid state imaging unit, wherein 23 shows the imaging unit as denoted by the dog line and the remaining numbers show components similar to those in FIG. 1. There are additionally provided second gate electrodes 24 and 26 between the first gate electrodes 14 and 25 in the adjoining imaging elements. Through the above mentioned procedure charges read in the first gate electrodes 14 moves below the second gate electrode 24 in the form of charge transfer upon the positive transfer pulse as in FIG. 1 applied to the second gate electrode 24. The charges below the second gate electrode 24 are sequentially transferred to the first gate electrode 25 and then second gate electrode 26 under the same operating principle and eventually to the output stage. In other words, it is possible to transfer the electric signals converted from the optical ones to the output stage using two-phase clock signals.
The DC bias voltage exerted on the second electrodes offers significant advantages as follows: One of those advantages is prevention of blooming. The blooming is the phenomenon in which light-generated carriers extend horizontally and the light-generated signals go beyond the range of the incident light in particular when the incident light is highly intensive. This developes a white line along the direction of transfer from a functional point of view in the case of the charge transfer type of solid state imagers. As is best shown in FIG. 2, the potential V.sub.CR at the diode which is held at V.sub.CR with the read pulse would decrease to V.sub.S during a frame interval because of light radiation. If the incident light is too intensive, then the diode potential would fall below V.sub.R as shown by the dot-and-dash line in FIG. 2b so that a channel below the first gate electrode may become conductive even when the transfer pulse is applied. As a result of this, current runs from the n.sup.+ region 13 to the n.sup.+ region 14 and pixels in the direction of transfer look like providing optical signals and cause fluctuations in the potential, developing a white line. However, provided that the electrode 20 overlying the photoconductor in FIG. 1 is supplied with a positive bias voltage V.sub.D 21, the diode potential never falls below V.sub.D even with the incident light. V.sub.D selected above V.sub.R allows no reading procedure responsive to the transfer pulse and alleviates the blurring phenomenon.
The approach to alleviate the blooming, however, presents outstanding problems as follows.
The above approach is not successful in suppressing the blooming and rather results in a decrease in signal components and in other words in the dynamic range.
V.sub.sat in FIG. 2 represents a voltage component corresponding to the amplitude of a signal when intervals light is incident.
This value V.sub.sat is considered as one which becomes saturated because of the intensive incident light and can be written as follows: EQU V.sub.sat -V.sub.CR -V.sub.C
As stated previously, the diode potential is selected such that it never falls below V.sub.R when intensive light impinges thereon during the period of light accumulation and therefore is correlated as V.sub.D &gt;V.sub.R.
According as the diode potential V.sub.D decreases, carriers (electrons in this case) activated by an optical signal corresponding to this potential penetrate into a diode disposed near the diode in issue and a transfer channel depletion layer via the substrate, thus resulting in the blooming phenomenon.
It is impossible to keep completely non-conductive the channel below the first gate electrode when the transfer pulse is applied, even though the DC source V.sub.D is connected to hold the diode potential substantially at V.sub.D.
This is because the signal carriers accumulated in the diode diffuse partly into the depletion layer due to a potential gradient as established by a potential at the adjoining diode or the transfer channel depletion layer.
The greater the quantity of the signal carriers which diffuse into the surrounding regions and result in the occurrence of the blooming (this is referred to as "excess carriers" hereinafter), the lower the diode potential V.sub.D or the stronger the incident light.
This implies as follows: Any attempt to suppress the quantity of the excess carriers demands an increase in the voltage value of the DC source (substantially equal to the diode potential). However, this approach entails a drop in the signal component V.sub.sat or the amplitude of the saturated signal, thus limiting the dynamic range.
Furthermore, with V.sub.D increased, burning takes place on the photoconductor layer as proved by the results of experiments.